Lithium ion secondary batteries have been widely used as a power supply to portable and compact equipment because of their output capability. More than a decade has passed since lithium secondary batteries were put on the market, and efforts to improve their performance have been continued. A high capacitance and safety are important technical targets for lithium secondary batteries.
In the prior art, the electrode of the lithium ion secondary battery is prepared by dispersing an electrode active material, a binder and optionally, a conductive agent in a solvent to form a slurry, and coating the slurry to a current collector typically in the form of a metal foil. The conductive agent is optionally added to the electrode and generally selected from among graphite, carbon black, acetylene black, carbon fibers, and metals such as nickel, aluminum, copper and silver, with graphite, carbon black and acetylene black being preferred.
The positive electrode active material for lithium ion secondary batteries is generally a material capable of taking lithium ions into and out of its structure. Examples include lithium-containing metal oxides such as lithium cobaltate, lithium nickelate, and lithium manganate, and lithium-containing composite metal oxides in which at least one metal element such as aluminum, manganese, tin, iron, copper, magnesium, titanium, zinc or molybdenum is added to the foregoing metal oxides.
These metal oxides, however, are short of electronic conduction and need the addition of conductive agents in order to use metal oxides as electrodes in lithium ion secondary batteries.
The preferred conductive agents are carbon black and acetylene black which can form a good conduction network. When they are used as the conductive agent for the positive electrode, the electrode is prepared by dispersing the conductive agent in a binder solution together with an active material to form a coating solution, and applying the coating solution to a metal foil, typically aluminum foil as a current collector, followed by drying and rolling.
In the step of drying the coated electrode, carbon black or acetylene black and the binder come afloat near the surface of the coating because of their low specific gravity, as the solvent evaporates off. Such segregation interferes with good conduction paths created by intimate contact among the active material, conductive agent and binder and adversely affects the adhesion between the current collector and the active material. This influence becomes more outstanding as the electrode thickness increases. For this reason, in the prior art positive electrode, the amount of active material laden per positive electrode unit area must be limited to 20 mg/cm2 or less. This becomes a bar against the desire to increase the energy density and output of lithium ion secondary batteries.
The prior art method for the preparation of electrodes by the wet process has the problem that the conductive agent and the binder can move apart from the collector foil during the drying step after the coating step, detracting from electrical conduction and adhesion to the collector. This phenomenon becomes outstanding particularly when an electrode active material having poor electronic conduction is used and a thick film electrode is to be formed. It is a serious barrier against acquiring satisfactory battery characteristics. Specifically, the phenomenon becomes outstanding when the positive electrode of a lithium ion secondary battery uses the above-mentioned positive electrode active material and the amount of active material laden per unit area is more than 20 mg/cm2. If the amount of active material laden per unit area exceeds 30 mg/cm2, the battery characteristics lower below the practically acceptable level.
This is also true, for example, when carbonaceous materials or titanium oxides capable of occluding and releasing lithium ions are used as the negative electrode active material and when vanadium oxides are used as the positive electrode active material. To solve the above problem, a better conduction path than in the prior art electrode must be created within the electrode material.